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The goal of cellular respiration and metabolism in animals and plants is,
ultimately, the conversion of one type of energy source to another. Presumably,
the original energy source comes in a form that cannot be immediately used to
support cellular activities. For humans, our external energy sources are the
foods we eat. Once we ingest and digest the food, our cells metabolic processes
convert the energy contained within the food into a form of energy that can
function in our cells. These constant conversions are what allow us to perform
our day-to-day activities.
Since energy is the ultimate goal of metabolism, it will be helpful to
understand what these various external and internal energy sources really are.
As we have mentioned, food is the external energy source for humans. Different
foods are composed primarily of one of the following three macromolecules:
carbohydrates (breads and pastas), lipids (fats and oils), or
proteins (meats and beans). During digestion of food, when the food is
first broken down internally, these large molecules are broken into subunits.
Depending on their type, subunits can be metabolized in different ways and then
used as internal energy sources.
The distinct means of metabolizing specific subunits all have the same goal, the
production of the primary cellular energy source: adenosine triphosphate.
Figure %: Chemical structure of ATP
As you can see in the figure above, ATP contains three phosphate groups. These
groups are primarily responsible for ATP's role as an energy source. During
metabolic reactions, these phosphate groups can be transferred from ATP to yield
either adenosine diphosphate (ADP) or adenosine monophosphate
(AMP).
ATP -> ADP + P + energy, or
ATP -> AMP + 2P + energy
The release of one or more phosphate groups is energetically favorable: the
reaction produces energy. ATP can also undergo a reaction with water to yield
ADP or AMP to release energy. The cell can use the energy produced from the
breakdown of ATP for whatever purpose is necessary. Often, the energetically
favorable breakdown of ATP is often coupled with another, energetically
unfavorable reaction that is designed to drive the first reaction forward
through the synthesis of additional ATP.
ATP synthesis is almost exactly opposite to the process by which ATP is broken
down to produce energy: phosphate groups are brought in contact with either ADP
or AMP. While this process is not as favorable, it is able to occur with the
energy derived from metabolizing foods. In addition to ATP, there are a number
of other reactive molecules that are involved in the production of cellular
energy. These are called coenzymes and their role is to help transfer other
chemical groups like hydrogens. Coenzymes work in conjunction with metabolic
enzymes to drive metabolic reactions. Among these are nicotinamide adenine
dinucleotide (NADH) and acetyl coenzyme A. We will discuss the specific
roles of both these molecules more in following sections.
